Vaios have their own proprietory software that you need to install in order for your Fn keys to function.
The problem with non-functioning Fn keys usually comes after a recent reinstall of Windows.
Simply go to the Vaio computers website of the country where you bought your laptop and enter the support section. Next, follow the steps (enter model number etc) in order to navigate to your laptop page where you will find driver and software downloads.
The software that you need should be listed as "Fn/function keys" or something similar.